arXiv subjects

Xinwu Cao

Publications and source records attributed to Xinwu Cao.

At least 55 records · Page 3Linked to original sources

arXiv2021

The large-scale magnetic field advected in the corona of a thin accretion disk

Large-scale magnetic field is believed to play a key role in launching and collimating jets/outflows. It was found that advection of external field by a geometrically thin disk is rather inefficient, while the external weak field may be dragged inwards by fast radially moving tenuous or/and hot gas above the thin disk. We investigate the field advection in a thin (cold) accretion disk covered with hot corona, in which turbulence is responsible for the angular momentum transfer of the gas in the disk and corona. The radial velocity of the gas in the corona is significantly higher than that in the thin disk. Our calculations show that the external magnetic flux is efficiently transported inwards by the corona, and the field line is strongly inclined towards the disk surface, which help launching outflows. The field configurations are consistent with those observed in the numerical simulations. The strength of the field is substantially enhanced in the inner region of the disk (usually several orders of magnitude higher than the external field strength), which is able to drive a fraction of gas in the corona into outflows. This mechanism may be useful in explaining the observational features in X-ray binaries and active galactic nuclei. Our results may help understanding the physics of the magneto-hydrodynamic (MHD) simulations.

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arXiv2019

Jets in the soft state in Cyg X-3 caused by advection of the donor magnetic field and unification with low-mass X-ray binaries

The high-mass accreting binary Cyg X-3 is distinctly different from low-mass X-ray binaries (LMXBs) in having powerful radio and $γ$-ray emitting jets in its soft spectral state. However, the transition from the hard state to the soft one is first associated with quenching of the hard-state radio emission, as in LMXBs. The powerful soft-state jets in Cyg X-3 form, on average, $\sim$50 d later. We interpret the initial jet quenching as due to the hard-state vertical magnetic field quickly diffusing away in the thin disc extending to the innermost stable circular orbit in the soft state, or, if that field is produced in situ, also cessation of its generation. The subsequent formation of the powerful jets occurs due to advection of the magnetic field from the donor. We find this happens only above certain threshold accretion rate associated with appearance of magnetically driven outflows. The $\sim$50 d lag is of the order of the viscous timescale in the outer disc, while the field advection is much faster. This process does not happen in LMXBs due to the magnetic fluxes available from their donors being lower than that for the wind accretion from the Wolf-Rayet donor of Cyg X-3. In our model, the vertical magnetic field in the hard state, required to form the jets both in Cyg X-3 and LMXBs, is formed in situ rather than advected from the donor. Our results provide a unified scenario of the soft and hard states in both Cyg X-3 and LMXBs.

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arXiv2019

Growth of massive black holes at high-z via accretion predominantly driven by magnetic outflows

Luminous quasars powered by accreting supermassive black holes (SMBHs) have been found in the early Universe at $z \gtrsim 7.5$, which set a strong constraint on both the seed black hole mass and the rapid growth of the SMBHs. In this work, we explore how the SMBHs are grown through Eddington limited accretion driven predominantly by magnetic outflows. Most angular momentum and the released gravitational energy in the disk can be removed by magnetic outflows, and therefore the mass accretion rate of the black hole (BH) can be high even if the disk is radiating at sub-Eddington luminosity. It is found that the SMBH with several billion solar masses discovered at $z\gtrsim 7$ may probably be grown through chaotic accretion predominantly driven by magnetic outflows from a stellar mass BH, when the disks are radiating at moderate luminosity ($\sim 0.5$ Eddington luminosity) with mild outflows. We find that most SMBHs are spinning at moderate values of spin parameter $a_*$, which implies only a small fraction of quasars may have radio jets.

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arXiv2019

A global solution to a slim accretion disk with radiation-driven outflows

The thickness of a slim disk is determined by the balance between the radiation force and the vertical component of the gravity of the black hole (BH). It was found that vertical gravity increases with the disk height, and it will decrease with the disk height if the disk thickness is above a critical value, which implies that gas at the disk surface may be driven into outflows by radiation force when the disk thickness surpasses the critical value. In this work, we derive a global solution to a slim disk with radiation-driven outflows. We find that the outflows are driven from the disk surface if the mass accretion rate $\dot{m} \gtrsim 1.78-1.91$ ($\dot{m}=\dot{M}/\dot{M}_{\rm Edd}$, and $\dot{M}_{\rm Edd}=L_{\rm Edd}/0.1c^2$) depending on BH mass, while the outflows are suppressed when the mass accretion rate is lower than this critical value. The mass accretion rate decreases with decreasing radius in the disk with outflows, and the rate of the gas swallowed by the BH is always limited to $\dot{m}_{\rm in}\sim 1.78-1.91$ even if the mass accretion rate at the outer edge of the disk is very high. This may set constraints on the massive BH growth through accretion in the early Universe. Due to the presence of outflows, there is an upper limit on the radiation flux of the disk, which leads to saturation of the continuum spectra of the disk with outflows at the high energy band. This may be tested by the observations of quasars or/and BH X-ray binaries.

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arXiv2019

Jet production in black-hole X-ray binaries and active galactic nuclei: mass feeding and advection of magnetic fields

Relativistic jets are observed only in the low/hard and intermediate states of X-ray binaries (XRBs), and are switched off in the thermal state, but they appear to be present in both low-luminosity and luminous active galactic nuclei (AGNs). It is widely believed that strong large-scale magnetic fields is a crucial ingredient in jet production; such fields can be attained only through efficient advection from the outer disc. We suggest that geometrically thin accretion discs with magnetic outflows are present in luminous radio-loud AGNs; this is likely because the interstellar medium provides both mass and sufficient magnetic flux to the outer disc. Most angular momentum of such disc is removed by the outflows, and the radial velocity of the disc is significantly increased compared to viscous drift velocity. This facilitates efficient magnetic field advection through the disc to produce a strong field near the black hole in luminous AGNs, which helps launch relativistic jets. In XRBs, the magnetic fields of the gas from companion stars are too weak to drive outflows from outer discs. Jets are therefore switched off in the thermal state due to inefficient magnetic field advection in the disc.

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arXiv2019

The large-scale magnetic field of a thin accretion disk with outflows

The large-scale magnetic field threading an accretion disk plays an important role in launching jets/outflows. The field may probably be advected inwards by the plasma in the accretion disk from the ambient environment (interstellar medium or a companion star). It has been suggested that the external field can be efficiently dragged inwards in a thin disk with magnetic outflows. We construct a self-consistent global disk-outflow model, in which the large-scale field is formed by the advection of the external field in the disk. The outflows are accelerated by this field co-rotating with the disk, which carry away most angular momentum of the disk and make its structure significantly different from the conventional viscous disk structure. We find that the magnetic field strength in the inner region of the disk can be several orders of magnitude higher than the external field strength for a geometrically thin disk with $H/R \sim 0.1$, if the ratio of the gas to magnetic pressure $β_{\rm out} \sim 10^2 $ at the outer edge of the disk. The outflow velocity shows layer-like structure, i.e., it decreases with radius where it is launched. The outflow can be accelerated up to $ \sim 0.2-0.3$c from the inner region of the disk, and the mass loss rate in the outflows is $ \sim 10 - 70\%$ of the mass accretion rate at the outer radius of the disk, which may account for the fast outflows observed in some active galactic nuclei (AGNs).

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arXiv2018

A failed-outflow model for the UV/optical blackbody emission in tidal disruption events

The temperature remains nearly constant while blackbody emission in UV/optical bands declines more than one order of magnitude in some tidal disruption events (TDEs). The physics behind it is still a mystery. A strong outflow can be driven by the radiation of the disc with super-Eddington luminosity. The disc emission drops rapidly to sub-Eddington luminosity, and the gas may fall back to the black hole. An optically thick shell is formed with gas temperature $\la 5\times 10^4$K due to line absorption, which is irradiated by the disc, and is re-emitting UV/optical photons. As the shell moves inwards, the gas at the inner surface of the shell is completely ionized at a certain temperature $\sim 10^4-5\times 10^4$K, which makes the gas optically thin for line absorption, and it therefore falls from the shell. This line absorbing layer acts as temperature regulator, i.e., the gas in the inner shell surface is removed to reduce the shell mass while the temperature is rising, and the decrease of the shell mass (gravity) decelerates the shell till a new balance is achieved between the radiation force and the gravity. This failed-outflow model can naturally explain the declining UV/optical blackbody radiation with constant temperature.

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arXiv2017

The power of the jets accelerated by the coronal magnetic field

It was suggested that the large scale magnetic field can be dragged inwards efficiently by the corona above the disc, i.e., the so called "coronal mechanism" (Beckwith, Hawley, \& Krolik 2009), which provides a way to solve the difficulty of field advection in a geometrically thin accretion disc. In this case, the magnetic pressure should be lower than the gas pressure in the corona. We estimate the maximal power of the jets accelerated by the magnetic field advected by the corona. The Blandford-Payne (BP) jet power is found always to be higher than the Blandford-Znajek (BZ) jet power, except for a rapidly spinning black hole with a>0.8. The maximal jet power is always low, less than 0.05 Eddington luminosity, even for an extreme Kerr black hole, which is insufficient for the observed strong jets in some blazars with jet power $\sim 0.1-1$ Eddington luminosity (or even higher). It implies that these powerful jets cannot be accelerated by the coronal field. We suggest that, the magnetic field dragged inward by the accretion disc with magnetically outflows may accelerate the jets (at least for the most powerful jets, if not all) in the blazars.

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arXiv2016

On the radio dichotomy of active galactic nuclei

It is still a mystery why only a small fraction of active galactic nuclei (AGNs) contain relativistic jets. Strong magnetic field is a necessary ingredient for jet formation, however, the advection of the external field in a geometrically thin disk is inefficient. The gas with a small angular velocity may fall from the Bondi radius $R_{\rm B}$ nearly freely to the circularization radius $R_{\rm c}$, and a thin accretion disk is formed within $R_{\rm c}$. We suggest that the external magnetic field is substantially enhanced in this region, and the magnetic field at $R_{\rm c}$ can be sufficiently strong to drive outflows from the disk if the angular velocity of the gas is low at $R_{\rm B}$. The magnetic field is efficiently dragged in the disk, because most angular momentum of the disk is removed by the outflows that leads to a significantly high radial velocity. The strong magnetic field formed in this way may accelerate jets in the region near the black hole either by the Blandford-Payne or/and Blandford-Znajek mechanisms. We suggest that the radio dichotomy of AGNs predominantly originates from the angular velocity of the circumnuclear gas. An AGN will appear as a radio-loud (RL) one if the angular velocity of the circumnuclear gas is lower than a critical value at the Bondi radius, otherwise, it will appear as a radio-quiet (RQ) AGN. This is supported by the observations that RL nuclei are invariably hosted by core galaxies. Our model suggests that the mass growth of the black holes in RL quasars is much faster than that in RQ quasars with the same luminosity, which is consistent with the fact that the massive black holes in RL quasars are systematically a few times heavier than those in their RQ counterparts.

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arXiv2016

On the thermal line emission from the outflows in ultraluminous X-ray sources

The atomic features in the X-ray spectra of the ultraluminous X-ray source (ULX) may be associated with the outflow (Middleton et al. 2015), which may provide a way to explore the physics of the ULXs. We construct a conical outflow model, and calculate the thermal X-ray Fe emission lines from the outflows. Our results show that thermal line luminosity decreases with increasing outflow velocity or/and opening angle of the outflow for a fixed kinetic power of the outflows. Assuming the kinetic power of the outflows to be comparable with the accretion power in the ULXs, we find that the equivalent width can be several eV for the thermal X-ray Fe emission line from the outflows in the ULXs with stellar mass black holes. The thermal line luminosity is proportional to 1/M (M is the black hole mass of the ULX). The equivalent width decreases with the black hole mass, which implies that the Fe line emission from the outflows can hardly be detected if the ULXs contain intermediate mass black holes. Our results suggest that the thermal X-ray Fe line emission should be preferentially be detected in the ULXs with high kinetic power slowly moving outflows from the accretion disks surrounding stellar mass black holes/neutron stars. The recent observed X-ray atomic features of the outflows in the ULX may imply that it contains a stellar mass black hole/neutron star (Pinto et al. 2016).

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arXiv2016

An accretion disk-outflow model for hysteretic state transition in X-ray binaries

We suggest a model of the advection dominated accretion flow (ADAF) with magnetically driven outflows to explain hysteretic state transition observed in X-ray binaries (XRBs). The transition from a thin disk to an ADAF occurs when the mass accretion rate is below a critical value. The critical mass accretion rate for the ADAF can be estimated by equating the equilibration timescale to accretion timescale of the ADAF, which is sensitive to its radial velocity. The radial velocity of thin disks is very small, which leads to the advection of the external field in thin disks very inefficient. ADAFs are present in the low/hard states of XRBs, and their radial velocity is large compared with the thin disk. The external field can be dragged inward efficiently by the ADAF, so a strong large-scale magnetic field threading the ADAF can be formed, which may accelerate a fraction of gas in the ADAF into the outflows. Such outflows may carry away a large amount of angular momentum from the ADAF, which increases the radial velocity of the ADAF significantly. This leads to a high critical mass accretion rate, below which an ADAF with magnetic outflows can survive. Our calculations show that the critical luminosity of the ADAF with magnetic outflows can be one order of magnitude higher than that for a conventional ADAF, if the ratio of gas to magnetic pressure $β\sim 4$ in the disk. This can naturally explain the hysteretic state transition observed in XRBs.

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arXiv2015

Constraints on black hole spins with a general relativistic accretion disk corona model

The peaks of the spectra of the accretion disks surrounding massive black holes in quasars are in the far-UV or soft X-ray band, which are usually not observed. However, in the disk corona model, the soft photons from the disk are Comptonized to high energy in the hot corona, and the hard X-ray spectra (luminosity and spectral shape) contain the information of the incident spectra from the disk. The values of black hole spin parameter $a_{\ast}$ are inferred from the spectral fitting, which spread over a large range, $\sim -0.94$ to $0.998$. We find that the inclination angles and mass accretion rates are well determined by the spectral fitting, while the results are sensitive to the accuracy of black hole mass estimates. No tight constraints on the black hole spins are achieved, if the uncertainties of black hole mass measurements are a factor of four, which are typical for the single-epoch reverberation mapping method. Recently, the accuracy of black hole mass measurement has been significantly improved to $0.2-0.4$~dex with velocity resolved reverberation mapping method (Pancoast et al., 2014). The black hole spin can be well constrained if the mass measurement accuracy is $\la 50$\%. In the accretion disk corona scenario, a fraction of power dissipated in the disk is transported into the corona, and therefore the accretion disk is thinner than a bare disk for the same mass accretion rate, because the radiation pressure in the disk is reduced. We find that the thin disk approximation, $H/R\la 0.1$, is still valid if $0.3<\dot{m}<0.5$, provided a half of the dissipated power is radiated in the corona above the disk.

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arXiv2015

The extension of variability properties in gamma-ray bursts to blazars

Both gamma-ray bursts (GRBs) and blazars have relativistic jets pointing at a small angle from our line of sight. Several recent studies suggested that these two kinds of sources may share similar jet physics. In this work, we explore the variability properties for GRBs and blazars as a whole. We find that the correlation between minimum variability timescale (MTS) and Lorentz factor, $Γ$, as found only in GRBs by Sonbas et al. can be extended to blazars with a joint correlation of $\rm MTS\proptoΓ^{-4.7\pm0.3}$. The same applies to the $\rm MTS\propto \it L_γ^{\rm -1.0\pm0.1}$ correlation as found in GRBs, which can be well extended into blazars as well. These results provide further evidence that the jets in these two kinds of sources are similar despite of the very different mass scale of their central engines. Further investigations of the physical origin of these correlations are needed, which can shed light on the nature of the jet physics.

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arXiv2015

Limits on luminosity and mass accretion rate of a radiation pressure dominated accretion disc

There is a maximum for the gravity of a black hole in the vertical direction in the accretion disc. Outflows may probably be driven from the disc if the radiation flux of the disc is greater than a critical value corresponding to the maximal vertical gravity. We find that outflows are driven by the radiation force from the disc if the accretion rate is greater than the Eddington rate. The radiation of the disc is therefore limited by such outflows. The disc luminosity, L=L_Edd\propto ln mdot, at large-mdot cases. The Eddington ratio of the disc is ~3 for mdot~100, which is significantly lower than that of a conventional slim disc without outflows. This implies that the emission from some ultra-luminous X-ray sources with highly super Eddington luminosity should be Doppler beamed, or intermediate mass black holes are in these sources instead of stellar mass black holes. The spectra of the discs with outflows are saturated in the high frequency end provided mdot>2. We suggest that the saturated emission can be observed to estimate the masses of the black holes accreting at high rates, such as the narrow-line Seyfert galaxies, with the model calculations. The rate of the mass accreted by the black hole is always around the Eddington rate even if the mass accretion rate at the outer radius is very high, because most of the gas is removed into the outflows. This implies that the luminous quasars at high redshifts z>6 should have grown up through persistent accretion at a rate close to the Eddington rate.

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arXiv2014

Episodic jet power extracted from a spinning black hole surrounded by a neutrino-dominated accretion flow in gamma-ray bursts

It was suggested that the relativistic jets in gamma-ray bursts (GRBs) are powered via the Blandford-Znajek (BZ) mechanism or the annihilation of neutrinos and anti-neutrinos from a neutrino cooling-dominated accretion flow (NDAF). The advection and diffusion of the large-scale magnetic field of a NDAF is calculated, and the external magnetic field is found to be dragged inward efficiently by the accretion flow for a typical magnetic Prandtl number P_m=1. The maximal BZ jet power can be ~10^53-10^54 erg/sec for an extreme Kerr black hole, if an external magnetic field with 10^14 Gauss is advected by the NDAF. This is roughly consistent with the field strength of the disk formed after a tidal disrupted magnetar. The accretion flow near the black hole horizon is arrested by the magnetic field if the accretion rate is below than a critical value for a given external field. The arrested accretion flow fails to drag the field inward and the field strength decays, and then the accretion re-starts, which leads to oscillating accretion. The typical timescale of such episodic accretion is in an order of one second. This can qualitatively explain the observed oscillation in the soft extend emission of short-type GRBs.

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arXiv2014

A toy model for X-ray spectral variability of active galactic nuclei

The long term X-ray spectral variability of ten active galactic nuclei (AGN) shows a positive spectral index-flux correlation for each object (Sobolewska & Papadakis 2009). An inner advection dominated accretion flow (ADAF) may connect to a thin disc/corona at a certain transition radius, which are responsible for hard X-ray emission in AGN. The ADAF is hot and its X-ray spectrum is hard, while the corona above the disc is relatively cold and its X-ray spectrum is therefore soft. The radiation efficiency of the ADAF is usually much lower than that of the thin disc. The increase of the transition radius may lead to decreases of the spectral index (i.e., a hard spectrum) and the X-ray luminosity even if the accretion rate is fixed, and vice versa. We propose that such X-ray variability is caused by the change of the transition radius. Our model calculations can reproduce the observed index-flux correlations, if the transition radius fluctuates around an equilibrium position, and the radiation efficiency of ADAFs is » 5 per cent of that for a thin disc. The average spectral index-Eddington ratio correlation in the AGN sample can also be reproduced by our model calculations, if the equilibrium transition radius increases with decreasing mass accretion rate.

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arXiv2014

The outflows accelerated by the magnetic fields and radiation force of accretion disks

The inner region of a luminous accretion disk is radiation pressure dominated. We estimate the surface temperature of a radiation pressure dominated accretion disk, Θ=(c_s/rΩ_K)^2<<(H/r)^2, which is significantly lower than that of a gas pressure dominated disk, Θ(H/r)^2. This means that the outflow can be launched magnetically from the photosphere of the radiation pressure dominate disk only if the effective potential barrier along the magnetic field line is extremely shallow or no potential barrier is present. For the latter case, the slow sonic point in the outflow may probably be in the disk, which leads to a slow circular dense flow above the disk. This implies that hot gas (probably in the corona) is necessary for launching a jet from the radiation pressure dominated disk, which provides a natural explanation on the observational evidence that the relativistic jets are related to hot plasma in some X-ray binaries and active galactic nuclei. We investigate the outflows accelerated from the hot corona above the disk by the magnetic field and radiation force of the accretion disk. We find that, with the help of the radiation force, the mass loss rate in the outflow is high, which leads to a slow outflow. This may be the reason why the jets in radio-loud narrow-line Seyfert galaxies are in general mild relativistic compared with those in blazars.

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arXiv2013

A Physical Link Between Jet Formation and Hot Plasma in Active Galactic Nuclei

Recent observations suggest that in black hole X-ray binaries jet/outflow formation is related to the hot plasma in the vicinity of the black hole, either in the form of an advection-dominated accretion flow at low accretion rates or in a disk corona at high accretion rates. We test the viability of this scenario for supermassive black holes using two samples of active galactic nuclei distinguished by the presence (radio-strong) and absence (radio-weak) of well-collimated, relativistic jets. Each is centered on a narrow range of black hole mass but spans a very broad range of Eddington ratios, effectively simulating, in a statistical manner, the behavior of a single black hole evolving across a wide spread in accretion states. Unlike the relationship between the radio and optical luminosity, which shows an abruptly break between high- and low-luminosity sources at an Eddington ratio of ~1, the radio emission-a measure of the jet power-varies continuously with the hard X-ray (2-10 keV) luminosity, roughly as L_R \propto L_X^(0.6-0.75). This relation, which holds for both radio-weak and radio-strong active galaxies, is similar to the one seen in X-ray binaries. Jet/outflow formation appears to be closely linked to the conditions that give rise to the hot, optically thin coronal emission associated with accretion flows, both in the regime of low and high accretion rates.

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